The present invention relates to the field of touch sensors, for example touch sensors for overlying a display screen to provide a touch-sensitive display (touch screen). In particular, embodiments of the invention relate to designs for electrode patterns for such sensors for sensing the presence of one or more touching objects within a two-dimensional sensing area.
A capacitive touch sensor can be generalised as one that uses a physical sensor element comprising an arrangement of electrically conductive electrodes extending over a touch sensitive area (sensing area) to define sensor nodes and a controller chip connected to the electrodes and operable to measure changes in the electrical capacitance of each of the electrodes or the mutual-capacitance between combinations of the electrodes. The electrodes are typically provided on a substrate. In some configurations electrodes are provided on both sides of a substrate, and these may be referred to as two-sided (two-layer) designs. In other configurations electrodes are provided on a single side of a substrate, and these may be referred to as single-sided (single-layer) designs. Single-sided designs are sometimes preferred because they have reduced manufacturing costs as compared to multi-layered designs. However, single-layer designs can be more challenging from a design point of view because of the restricted topology, principally because electrode interconnections cannot cross one another in a single plane.
FIG. 1 schematically shows some principal components of a generic two-sided capacitive touchscreen comprising a physical sensor element 100. The touch screen is represented in plan view (to the left in the figure) and also in cross-sectional view (to the right in the figure).
The touch screen is configured for establishing the position of a touch within a two-dimensional sensing area by providing Cartesian coordinates along an X-direction (horizontal in the figure) and a Y-direction (vertical in the figure). In this example the sensor element 100 is constructed from a substrate 103 that could be glass or plastic or some other insulating material and upon which is arranged an array of electrodes consisting of multiple laterally extending parallel electrodes, X-electrodes 101 (row electrodes), and multiple vertically extending parallel electrodes, Y-electrodes 102 (column electrodes), which in combination allow the position of a touch 109 to be determined. To clarify the terminology, and as will be seen from FIG. 1, the X-electrodes 101 (row electrodes) are aligned parallel to the X-direction and the Y-electrodes 102 (column electrodes) are aligned parallel to the Y-direction. Thus the different X-electrodes allow the position of a touch to be determined at different positions along the Y-direction while the different Y-electrodes allow the position of a touch to be determined at different positions along the X-direction. That is to say in accordance with the terminology used herein, the electrodes are named (in terms of X- and Y-) after their direction of extent rather than the direction along which they resolve position. Furthermore, the electrodes may also be referred to as row electrodes and column electrodes. It will however be appreciated these terms are simply used as a convenient way of distinguishing the groups of electrodes extending in the different directions. In particular, the terms are not intended to indicate any specific electrode orientation. In general the term “row” will be used to refer to electrodes extending in a horizontal direction for the orientations represented in the figures while the terms “column” will be used to refer to electrodes extending in a vertical direction in the orientations represented in the figures.
In some cases, each electrode may have a more detailed structure than the simple “bar” structures represented in FIG. 1, but the operating principles are broadly the same. The sensor electrodes are made of an electrically conductive material such as copper or Indium Tin Oxide (ITO). The nature of the various materials used depends on the desired characteristics of the touch screen. For example, a touch screen may need to be transparent, in which case ITO electrodes and a plastic substrate are common. On the other hand a touch pad, such as often provided as an alternative to a mouse in laptop computers is usually opaque, and hence can use lower cost copper electrodes and an epoxy-glass-fibre substrate (e.g. FR4). Referring back to FIG. 1, the electrodes are electrically connected via circuit conductors 104 to a controller chip 105, which is in turn connected to a host processing system 106 by means of a communication interface 107. The host 106 interrogates the controller chip 105 to recover the presence and coordinates of any touch or touches present on, or proximate to the sensor 103. In the example, a front cover (also referred to as a lens or panel) 108 is positioned in front of the sensor 103 and a single touch 109 on the surface of the cover 108 is schematically represented.
Note that the touch itself does not generally make direct galvanic connection to the sensor 103 or to the electrodes 102. Rather, the touch influences the electric fields 110 that the controller chip 105 generates using the electrodes 102. With appropriate analysis of relative changes in the electrodes' measured capacitance/capacitive coupling, the controller chip 105 can thus calculate a touch position on the cover's surface as an XY coordinate 111. The host system can therefore use the controller chip to detect where a user is touching, and hence take appropriate action, perhaps displaying a menu or activating some function.
There are many different material combinations and electrode configurations to allow creation of a touch screen and the example discussed above is just one.
A further aspect of capacitive touch sensors relates to the way the controller chip uses the electrodes of the sensor element to make its measurements. There are two main classes of controller in this regard.
A first class is based on measuring what is frequently referred to as “self-capacitance”. Reference is made to FIG. 2. In this design of a capacitive sensor, the controller 201 will typically apply some electrical stimulus (drive signal) 202 to each electrode 203 which will cause an electric field to form around it 204. This field couples through the space around the electrode back to the controller chip via numerous conductive return paths that are part of the nearby circuitry 205, product housing 206, physical elements from the nearby surroundings 207 etc., so completing a capacitive circuit 209. The overall sum of return paths is typically referred to as the “free space return path” in an attempt to simplify an otherwise hard-to-visualize electric field distribution. The important point to realise is that the controller is only driving each electrode from a single explicit electrical terminal 208; the other terminal is the capacitive connection via this “free space return path”. The capacitance measured by the controller is the “self-capacitance” of the sensor electrode (and connected tracks) relative to free space (or Earth as it is sometimes called) i.e. the “self-capacitance” of the relevant sensor electrode. Touching or approaching the electrode with a conductive element 210, such as a human finger, causes some of the field to couple via the finger through the connected body 213, through free space and back to the controller. This extra return path 211 can be relatively strong for large objects (such as the human body), and so can give a stronger coupling of the electrode's field back to the controller; touching or approaching the electrode hence increases the self-capacitance of the electrode. The controller is configured to sense this increase in capacitance. The increase is strongly proportional to the area 212 of the applied touch and is normally weakly proportional to the touching body's size (the latter typically offering quite a strong coupling and therefore not being the dominant term in the sum of series connected capacitances).
In a common two-layer self-capacitance sensor the electrodes are arranged on an orthogonal grid, generally with a first set of electrodes on one side of a substantially insulating substrate and the other set of electrodes on the opposite side of the substrate and oriented at nominally 90° to the first set. There are also structures where the grid is formed on a single side of the substrate and small conductive bridges are used to allow the two orthogonal sets of electrodes to cross each other without short circuiting. However, these designs are more complex to manufacture and less suitable for transparent sensors. There are also known designs where the electrode pattern is formed on a single side of a substrate and external connections are used to allow the respective electrodes to be appropriately connected, as discussed further below. One set of electrodes is used to sense touch position in a first axis that we shall call “X” and the second set to sense the touch position in the second orthogonal axis that we shall call “Y”.
In a self-capacitance touch sensor, the controller can either drive each electrode in turn (sequential) with appropriate switching of a single control channel or it can drive them all in parallel with an appropriate number of separate control channels. In the former sequential case, any neighbouring electrodes to a driven electrode are sometimes grounded by the controller to prevent them becoming touch sensitive when they are not being sensed (remembering that all nearby capacitive return paths will influence the measured value of the actively driven electrode). In the case of the parallel drive scheme, the nature of the stimulus applied to all the electrodes is typically the same so that the instantaneous voltage on each electrode is approximately the same. The drive to each electrode is electrically separate so that the controller can discriminate changes on each electrode individually, but the driving stimulus in terms of voltage or current versus time, is the same. In this way, each electrode has minimal influence on its neighbours (the electrode-to-electrode capacitance is non-zero but its influence is only “felt” by the controller if there is a voltage difference between the electrodes).
The second class of controller is based on measuring what is frequently referred to as “mutual-capacitance”. Reference is made to FIG. 3. In this design of a capacitive sensor the controller 301 will sequentially stimulate each of an array of transmitter (driven/drive) electrodes 302 that are coupled by virtue of their proximity to an array of receiver electrodes 303. The resulting electric field 304 is now directly coupled from the transmitter to each of the nearby receiver electrodes; the “free space” return path discussed above plays a negligible part in the overall coupling back to the controller chip when the sensor is not being touched. The area local to and centred on the intersection of a transmitter and a receiver electrode is typically referred to as a “node”. Now, on application or approach of a conductive element 305 such as a human finger, the electric field 304 is partly diverted to the touching object 305. An extra return path to the controller 301 is now established via the body 306 and “free-space” in a similar manner to that described above. However, because this extra return path acts to couple the diverted field directly to the controller chip 301, the amount of field coupled to the nearby receiver electrode 303 decreases. This is measured by the controller chip 301 as a decrease in the “mutual-capacitance” between that particular transmitter electrode and receiver electrodes in the vicinity of the touch. The controller senses this change in capacitance of one or more nodes. For example, if a reduction in capacitive coupling to a given Y-electrode is observed while a given X-electrode is being driven, it may be determined there is a touch in the vicinity of where the given X-electrode and given Y-electrode cross within the sensing surface. The magnitude of a capacitance change is nominally proportional to the area 307 of the touch (although the change in capacitance does tend to saturate as the touch area increases beyond a certain size to completely cover the nodes directly under the touch) and weakly proportional to the size of the touching body (for reasons as described above). The magnitude of the capacitance change also reduces as the distance between the touch sensor electrodes and the touching object increases.
In a common two-sided mutual-capacitance sensor the transmitter electrodes and receiver electrodes are arranged as an orthogonal grid, with the transmitter electrodes on one side of a substantially insulating substrate and the receiver electrodes on the opposite side of the substrate. This is as schematically shown in FIG. 3. In FIG. 3 a first set of transmitter electrodes 303 is shown on one side of a substantially insulating substrate 308 and a second set of receiver electrodes 302 is arranged at nominally 90° to the transmitter electrodes on the other side of the substrate. There are also structures where the grid is formed on a single side of the substrate and small insulating bridges, or as discussed below external connections, are used to allow the transmitter and receiver electrodes to be connected to in rows and columns without short circuiting.
By using interpolation between adjacent nodes for both types of capacitive touch sensor a controller chip can typically determine touch positions to a greater resolution than the spacing between electrodes. Also there are established techniques by which multiple touches within a sensing area, and which might be moving, can be uniquely identified and tracked, for example until they leave the sensing area.
For a touch sensor that is transparent, for example because it is intended to overlay a display the electrodes may be provided by a transparent conductor material, such as ITO. However, using transparent conductor materials can be relatively expensive, for example as compared to using copper. Accordingly, it has been proposed to define electrodes for a transparent conductor using a mesh of thin copper traces.
FIG. 4 schematically represents an arrangement of electrodes 34 on one side of a two-sided sensor 30, for example of the kind represented in FIGS. 1, 2 and 3, and in which the electrodes are defined by a mesh of thin copper traces on a substrate 32. Each electrode 34 is defined by an appropriately-shaped region (in this case horizontal bars) comprising a grid of copper wires. In this example the copper wire grid is angled at around 45 degrees to the horizontal/vertical directions and is made of copper wires having a width on the substrate of around 3 microns and a pitch of around 200 microns (the thickness of the copper layer will typically be around 0.1 to 2 microns). The touch sensor is overall transparent because of the copper wires cover only a relatively small fraction of the area. Furthermore, with relatively large area electrodes (i.e. large compared to the 200 microns pitch of the wire mesh) such as represented in FIG. 4, the wire mesh is barely visible to a user looking through the touch sensor at an underlying screen. That is to say, the wire mesh does not give rise to significant visual artefacts.
FIG. 5 schematically represents a conventional single-sided (single-layer) electrode pattern for a capacitive touch sensor 40. The sensor 40 comprises an array of sensing nodes 42 arranged in a plurality of rows and columns across a two-dimensional sensing surface. In this example there are five rows schematically labelled R1 to R5 (running horizontally for the orientation represented in the figure) and six columns schematically labelled C1 to C6 (running vertically for the orientation represented in the figure). Thus the sensing surface extends horizontally from a first (left) edge 47A adjacent column C1 to a second (right) edge 47B adjacent column C6 and extends vertically from a third (top) edge 47C adjacent row R1 to a fourth (bottom) edge 47D adjacent row R5.
Each sensing node 42 comprises a first electrode 43 and a second electrode 44. The first electrodes are schematically represented in FIG. 5 with darker shading than the second electrodes. A plurality of traces 45 connect respective ones of the first electrodes 43 to a perimeter of the sensing surface, in this case down to the fourth (bottom) edge 47D adjacent row R5. There is a separate trace 45 for each of the first electrodes 43. The respective first electrodes 43 of each row R1 to R5 are electrically connected together outside the surface of the sensing area by external wiring (not shown) connecting to the respective traces 45 at the perimeter of the sensing area. A plurality of further traces 46 interconnect respective ones of the second electrodes 44 in the same column, and the respective further traces also extend down to the perimeter of the sensing area along the fourth edge 47D. Ground traces 48 (schematically represented in FIG. 5 with dotted lines) are provided at locations where traces 45 connecting to the first electrodes 43 and further traces 46 connecting to the second electrodes 46 would otherwise be adjacent.
Thus, in the arrangement represented in FIG. 5 the first electrodes 43 in each row are interconnected (via their respective traces 45 and external wiring) and the second electrodes in each column are interconnected (by the further traces 46) within the sensing area. In this regard the arrangement of electrodes in FIG. 5 provides an array of interconnected rows and columns defining a two-dimensional array of sensing nodes. In effect the sensing nodes 43 of FIG. 5 correspond to the sensing nodes at the crossing points in the two-layer designs of FIGS. 1 to 3, but with electrodes provided on only a single layer of a substrate. Thus, the approach of FIG. 5 can be advantageous in certain circumstances, for example because of simpler manufacturing and/or higher transparency. The sensing element represented in FIG. 5 can be connected to conventional drive circuitry for establishing the position of an object adjacent the sensing surface in accordance with conventional techniques such as discussed above with reference to FIGS. 1 to 3. Thus the sensing element can be used in a mutual-capacitance mode in which capacitive coupling between the respective first electrodes and the second electrodes are measured to identify which sensing nodes are associated with a change in mutual capacitance caused by a proximate object. The sensing elements can also be used in a self-capacitance mode in which the self-capacitance of the respective electrodes are separately measured to identify which sensing nodes are associated with a change in mutual capacitance. In this regard, the interconnection of the electrodes into rows and columns provides a matrix approach which reduces the number of control channels required (as compared to approaches where the individual sensing nodes are coupled to separate measurement channels).
Whilst a single-layer design of the type represented in FIG. 5 can be advantageous from a manufacturing point of view, it requires relatively thin trace circuitry to avoid overly-large areas of insensitivity between columns. For example, the traces 45 connecting from the lower edge to the respective first electrodes 43 of the respective sensor nodes 42 in a given column in a typical implementations may be separated by only 100 microns or so. It is not possible to provide a group of such closely spaced electrodes using a wire-mesh arrangement of the kind represented in FIG. 4 with a pitch of around 200 microns, and simply using a regular grid having a smaller pitch size can be expected to reduce transparency to levels which are unacceptable for many touch-screen applications.
An alternative approach could be to simply replicate the pattern of FIG. 5 with linear electrodes, i.e. using an array of thin copper electrodes deposited on a substrate and extending generally in the vertical direction (for the orientation of in FIG. 5) with appropriate horizontal connections for defining the pattern as appropriate. However, the Inventor has recognised drawbacks with this approach in situations where the sensor is to overlay a display screen (i.e. where the position sensor is incorporated in a touchscreen). In particular, the Inventor has recognised the way in which the electrodes extending generally in the vertical direction of FIG. 5 interact with pixels in the underlying display screen can give rise to distracting effects such as moiré patterning and variations in apparent colour, both across the screen and for different viewing angles, as the electrodes obscure different portions of the underlying pixels.
There is therefore a desire to provide touch sensors with electrode patterns that can be implemented using non-transparent conductors in a single layer design with reduced visible artefacts.